Problem/Motivation
When a drupal core release comes out the accompanying translation files will not be available immediately. It takes at least a few hours (up to days) before translation files of the (point) release. During this time the installation of Drupal in a non English language fail with a requirements fault "The %language translation is not available."
Currently the installer (install_get_localization_release()) only provides a fallback in case the release is a development snapshot. For example 8.0-dev falls back to 7.0 and 8.2-dev falls back to 8.1.
The main problem we need to solve is the user experience. We need to make sure that a translation is found. Even a quite old translation would be acceptable. Once the Interface translation (local) and Update module are installed and enabled, a more intelligent update mechanism is available and more recent translations will be downloaded and applied.
Proposed resolution
* Implement simple (hardwired) fallback rules for core translation releases.
* Allow for multiple suggestions and multiple 'download' attempts.